1. Field of the Invention
The instant invention is directed to a catalyst useful in the polymerization of olefins, particularly homopolymerization and copolymerization involving ethylene. Specifically, the instant invention is directed to a catalyst comprised of a zirconium composition, a vanadium composition and an activator. The instant invention is particularly useful in producing an ethylene homopolymer having a broad molecular weight distribution (MWD), a bimodal molecular weight distribution profile, and a unique combination of physical properties, all in a single reactor.
2. Description of the Prior Art
The polymerization of olefins using transition metal catalysts is well established in the art. Polymerizations employing these catalysts produce polyolefins possessing desired characteristics in high yield, usually at processing conditions of low temperature and low pressure, thus making these catalysts the subject of much research. An especially important class of catalysts where improvement is sought is that class of catalysts which produce ethylenic and/or alpha-olefinic polymers, particularly the commercially important polymer, polyethylene.
Of practical interest is the development of a catalyst which will yield polymer adaptable for use in high strength films and light weight blow molding resins. Generally, these characteristics are indicative of a polymer having a broad molecular weight distribution. The usual measure of the dispersity of the molecular weight distribution in the art has been the ratio of the weight average molecular weight to the number average molecular weight. A distribution is said to be narrow if for a given polymer type of constant average molecular weight, the number average molecular weight is substantially the same as the weight average molecular weight. As the ratio of weight average molecular weight to number average molecular weight increases from over 1:1, the distribution becomes broad. Generally, as the distribution becomes broad, film strength and resin processability improve. Thus in applications where these traits are of concern, a broad molecular weight distribution is desirable.
To further modify the physical characteristics and performance properties of a polymer, the shape of the molecular weight distribution curve can be varied. One technique of varying the molecular weight so that it becomes broadened is by making the distribution polymodal, i.e., making the polymer appear as though it consists of two (bimodal) or three (trimodal) distinct polymers. By controlling the location and concentration of the modes, the molecular weight distribution may be varied and different processing and end-use properties may be obtained. To achieve these results special catalyst systems must be used.
To attain these properties, as desired, the common practice heretofore has entailed the use of multiple reactors, such as in a cascade system, and, as necessary, the employment of different catalysts in the reactor sequence so to produce the sought-after polymer characteristics. Because of the complexity and cost of such processing, as correlates for example to the number of reactors required, the necessity of different catalysts etc., the art recognizes a continuing need of producing the aforementioned polymer characteristics without multiple reactors or multiple catalysts.
Though desirable, the art has been unable to develop an entirely satisfactory catalyst able to produce, in a single reactor, a polymer having the requisite properties while at the same time evincing high activity, i.e., leaving low catalyst residue in the product, good hydrogen response (to facilitate control of molecular weight), minimization of low molecular weight tail so as to eliminate odor or smoke problems that often occur in downstream processing, and not requiring a substantial amount of promoter.